This invention relates generally to radiometric imagers that use a scanning interferometer, and, more particularly, to methods and apparatus for precisely measuring the velocity of an interferometer relative to a scene being scanned.
Radiometric imagers have numerous practical applications, including the mapping of terrestrial, celestial and oceanic features, the measuring of atmospheric water vapor, rain and sea surface temperature, and the assessment of hydrographic phenomena and surface conditions beneath clouds or rain.
One kind of radiometric imager, which uses a scanning interferometer, is described in a copending and commonly-assigned U.S. patent application, Ser. No. 607,848, filed May 7, 1984, in the name of Carl A. Wiley and entitled "Interferometric Radiometer." The radiometer disclosed in that copending application utilizes an interferometer having a fringe pattern with a series of sensitivity lobes of predetermined widths. As the interferometer scans a scene, it produces an interferometer signal based on the superimposed contributions of all of the emitters of radiation in the interferometer's field of view. The radio-frequency distribution of the scene along the direction of the scan can then be reconstructed using a predetermined signature or matched filter reference signal that corresponds to the interferometer signal that would result from the interferometer scanning a single, constant point emitter of radiation in the scene being scanned. Preferably, a fast Fourier transform of the interferometer signal is multiplied by the complex conjugate of a fast Fourier transform of the matched filter signal, after which the inverse Fourier transform of the resulting product is computed, to collapse the data. The collapsed data indicates the location and intensity of all point sources of radiation in the scene being scanned. Substantially similar results can be obtained by comparing the interferometer signal with the stored matched filter signal, in the time domain, using either convolution or cross-correlation.
The radiometric imager described in the copending application requires the use of a matched filter signal that corresponds precisely with the interferometer signal that would result from scanning a single, fixed emitter of radiation. Because the proper matched filter signal varies in accordance with the relative tangential velocity of the interferometer and the scene being scanned, however, proper imaging cannot be achieved without knowledge of the precise relative velocity. This tangential velocity corresponds to the true velocity of the interferometer relative to the scene divided by the range, i.e., V/H.
In the past, the relative tangential velocity of the interferometer and the scene being scanned has been determined using any of a number of alternative techniques, none of which has proven to be entirely satisfactory. One such technique measures the rate at which the interferometer's successive lobes are crossed by individual point emitters of radiation in the scene. Another technique utilizes triangulation to determine the change in the interferometer's position over a specified time period. Although techniques such as these for determining the relative velocity are generally effective, they are believed to be unduly complicated or not sufficiently accurate to enable creation of radiometric images of the desired resolution.
It should, therefore, be appreciated that there is a need for an improved method and apparatus for measuring the tangential velocity of an interferometer in a radiometric imager relative to a scene being scanned, that is not unduly complex or expensive and that provides a relative velocity measurement of high precision. The present invention fulfills this need.